A conventional RAM (Random Access Memory) cell consists of a transistor and a capacitor mostly made from Silicon dioxide (SiO2). The transistor is used to control the inflow and outflow of charge stored in the capacitor as the physical quantity exploitable for storing information. Said transistor also decouples the capacitors from each other. Such RAM cells, also termed Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM), have the disadvantage that information stored therein is volatile and as such is lost upon each power supply failure. Further, the time needed to refresh the information contained in conventional RAM cells delimits the read and write performance of such cells and causes a constant need of electrical power. Thus, a change in computer RAM technology beyond conventional, volatile, DRAM and SRAM would be desirable.
Alternative memory devices that retain their memory state even after removing the electrical power from the device, i.e., non-volatile memory, to replace conventional RAM have been proposed. In Magnetic Random Access Memory (MRAM), which is based on magnetic tunnel junctions, the relative orientation of two ferromagnetic layers is exploited to store digital information. In ferroelectric non-volatile RAM (FERAM) cells, the ferroelectric polarization of a bit storing layer is used to define two different states which can be associated with two different logical values. Another example is so-called phase change RAM (PCRAM). In this approach the medium, typically a chalcogenide compound, can be switched between a high-resistance amorphous and a low-resistance poly-crystalline state by a current pulse.
Yet another example for non-volatile memory device is a resistor with two (or more) reversibly switchable and persistent resistance-states. U.S. Pat. No. 6,204,139 describes a method for switching properties of transition metal oxide materials used in thin film resistors. The properties, in particular the resistance, are switched reversibly by short electrical pulses. Application of the method for non-volatile memory cells is proposed. Digital information is stored by, for example, associating a high-resistance state with a logic “0” and a low-resistance state with a logic “1”.
The articles “Reproducible switching effect in thin oxide films for memory applications” (A. Beck et al., Applied Physics letters, Vol. 77, No. 1, July 2000), “Current-driven insulator-conductor transition and non-volatile memory in chromium-doped SrTiO3 single crystals” (Y. Watanabe et al., Applied Physics Letters, Vol. 78, No. 23, June 2001), and “Electrical current distribution across a metal-insulator-metal structure during bistable switching” (C. Rossel et al., Journal of Applied Physics, Vol. 90, No. 6, September 2001) and the international application publication WO 00/49659 A1 describe materials and classes of materials with reversibly switchable resistance, and simple resistor devices made from these materials. The resistance states are persistent, i.e., non-volatile. Also multi-level switching is realized.
The above-mentioned resistance switching devices provide storage densities and manufacturing costs which are still unsatisfactory for many applications.